In the handling and transporting of compressed gases stored in cylinders, it is known that exposure of the cylinders to heat may cause the pressure inside the cylinder to increase. It is also known to provide a pressure-relief mechanism that will open to vent the gas to the surrounding environment if the pressure within the cylinder reaches levels that may cause the cylinder to rupture.
There are instances, however, where it is advantageous to prevent the contents of the cylinder from being released. One such instance involves the transportation of compressed oxygen cylinders in cargo holds. If the cargo hold is provided with flame retardant walls and is sealed, the fire may smolder or suffocate before causing any serious damage. In such a situation, it would be disadvantageous to allow the oxygen inside the cylinders to release into the area surrounding the cylinders because the oxygen may provide fuel for the fire. Another problematic situation involves the shipment of toxic gases in cylinders or oxygen generators in cargo holds. It is also known that high temperatures may cause inadvertent actuation of oxygen generators.
For shipping cylinders containing compressed oxygen, it has been known to provide an outer packaging or over pack to protect the cylinders from reaching temperatures that may trigger the pressure-relief mechanism. It has been determined that a container constructed to ATA 300 requirements provides a minimal amount of thermal protection to a cylinder exposed to a high temperature environment. The ATA 300 container is an over pack or outer packaging commonly used by airlines to transport their oxygen cylinders (ATA specification No. 300, Packaging of Airline Supplies, Category 1). An ATA Specification No. 300 Category 1 (ATA 300) over pack or outer packaging is a resilient, durable over pack intended to be reused for a minimum of 100 round trips.
The existing ATA 300 containers have several drawbacks due to their lack of thermal protection. Accordingly, there is a need for a thermally protected over pack container that extends the time before a pressure relief mechanism on a compressed oxygen cylinder is triggered when the cylinder is exposed to elevated temperatures.
Marker and Diaz in DOT/FAA/AR-TN98/30 entitled Evaluation of Oxygen Cylinder Over packs Exposed to Elevated Temperature describe a modified Viking Corporation over pack containing thermal insulation. The over pack exterior consists of a heat-resistant thermoplastic known as Kydex. A 1-inch-thick fiberglass batt material is sandwiched between the exterior layer of Kydex and an additional layer of Kydex of the same thickness. A layer of polyethylene foam is glued to the internal layer of Kydex to provide impact resistance. After test initiation, the furnace temperature reached 400° F. in 10 minutes. The test was allowed to progress for approximately 60 minutes, at which point large quantities of smoke began to appear from the test furnace vents. The temperature of the cylinder surface never exceeded 90° F. during the test. A post test inspection revealed the source of the smoke was from the two ends of the over pack which had come in contact with the furnace heating elements. The heated thermoplastic lost some of its structural integrity, allowing the ends to sag and eventually come in contact with the furnace surface. In addition, the latch mounts had pulled away from the over pack due to the rivets pulling through the heat-softened thermoplastic exterior, exposing the fiberglass insulation. The interior of the over pack was undamaged.
A subsequent test was performed on an upgraded version of the thermally protected over pack. The new design utilized aluminum-faced rigid insulating foam in place of the fiberglass batting. External and internal layers of Kydex surrounded the rigid foam. After loading the charged cylinder into the new-design over pack, the furnace was activated and the temperature approached 400° F. in approximately 15 minutes. During the test, the temperature of the cylinder surface reached a maximum of 210° F. at 90 minutes, at which point the test was terminated. A post test inspection revealed the external layer of Kydex had melted and burned in several locations, exposing the aluminum foil face of the rigid foam insulation panel which had remained intact. The inner layer of Kydex was slightly warped but had not changed color. Although the cylinder surface temperatures were kept relatively low, the cylinder and valve assembly had become slightly discolored as a result of combustion of the Kydex and possibly the rigid foam panel. Due to a malfunction with the data acquisition, the temperature versus time data obtained during the test could not be retrieved. However, these data were observed during the test and indicated a gradual increase of the cylinder surface temperature up to a maximum of 210° F. at 90 minutes. The primary disadvantage of these over packs was the outer container material was made of Kydex, a material not capable of withstanding direct exposure to flame. Thus, the Kydex eventually melted and/or ignited.
U.S. Pat. No. 6,595,383 describes a single use over pack and a method for packing a compressed gas cylinder in an over pack, which provides thermal protection that is the same as or better than the ATA 300 Class III. Unfortunately, this device has the drawbacks of single use, pollution causing flame resistant treated cardboard and inconvenient cylinder loading characteristics.